"warning: unnecessary lifetime parameter" unsilenceable lint

[ijackson]

Given the following code:

pub fn thing<'a>(_s: &'a str) where 'a: 'static { }

The current output is:

warning: unnecessary lifetime parameter `'a`
 --> src/lib.rs:2:37
  |
2 | pub fn thing<'a>(_s: &'a str) where 'a: 'static { }
  |                                     ^^
  |
  = help: you can use the `'static` lifetime directly, in place of `'a`

warning: `playground` (lib) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 1.27s

Ideally the output should also say something like this:

  = note: `#[warn(needless_lifetimes)]` on by default

And of course then #[allow(needless_lifetimes)] ought to work.

Versions affected

rustcargo@zealot:/volatile/rustcargo/Rustup/Game/server$ rustc -vV
rustc 1.62.0-nightly (ecd44958e 2022-05-10)
binary: rustc
commit-hash: ecd44958e0a21110d09862ee080d95a4ca6c52f8
commit-date: 2022-05-10
host: x86_64-unknown-linux-gnu
release: 1.62.0-nightly
LLVM version: 14.0.1
rustcargo@zealot:/volatile/rustcargo/Rustup/Game/server$ 

The example code is accepted without any complaint by stable and beta, However, in both stable and beta, clippy has the same weird name-less complaint. And saying #![allow(clippy::needless_lifetimes)] doesn't suppress the message.

I searched for "lifetime" in the clippy lints db. Of those that came up, needless_lifetimes and redundant_static_lifetimes seemed like they might be it, but neither of those suppresses the lint on stable's clippy.

[ijackson]

@rustbot modify labels +regresson-from-stable-to-nightly

[rustbot]

Error: Label regresson-from-stable-to-nightly can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.

[ijackson]

This is not a clippy lint. It is being emitted, here, by rustc:

rustcargo@zealot:~$ cat t.rs
pub fn thing<'a>(_s: &'a str) where 'a: 'static { }
fn main() { thing(""); }
rustcargo@zealot:~$ rustc +nightly t.rs
warning: unnecessary lifetime parameter `'a`
 --> t.rs:1:37
  |
1 | pub fn thing<'a>(_s: &'a str) where 'a: 'static { }
  |                                     ^^
  |
  = help: you can use the `'static` lifetime directly, in place of `'a`

warning: 1 warning emitted

rustcargo@zealot:~$ 

The situation is confusing because there is (or was?) also a clippy lint for this; I think this may have been promoted into rustc but something was done wrong there.

I have rechecked and:

• rustc 1.61.0 (fe5b13d68 2022-05-18) (stable) unaffected
• rustc 1.62.0-beta.2 (daf68b1f7 2022-05-19) affected
• rustc 1.63.0-nightly (4cbaac699 2022-05-25) affected

@rust-lang/release could you please set the regresson-from-stable-to-beta label for me?

[ijackson]

@rustbot ping rust-lang/release

[rustbot]

Error: Only Rust team members can ping teams.

Please let @rust-lang/release know if you're having trouble with this bot.

[ijackson]

@rustbot modify labels +regression-untriaged

[ijackson]

Ah! Looking at the "Regression" bug template gave me a hint.

[gimbling-away]

Ahh, I see. I was testing on stable, my bad.

[apiraino]

bisected the regression (with cargo-bisect) points to pr #93803 (cc @cjgillot )

searched nightlies: from nightly-2022-02-01 to nightly-2022-06-07
regressed nightly: nightly-2022-05-01
searched commit range: a707f40...7c4b476
regressed commit: d201c81

bisected with cargo-bisect-rustc v0.6.1

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start=2022-02-01 --script=./test.sh 

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-medium

[cjgillot]

#93803 made this warning more precise, so it triggers more often.
This is a hard-warning. Fixing it requires to turn it into a lint.

Steps:

• declare the lint in rustc_lint_defs::builtin and write a proper explanation;
• in rustc_resolve::late::lifetimes, replace the call to struct_span_warn by struct_span_lint_hir, the reference HirId is the bounded lifetime's one;
• (option) move the lint to rustc_typeck::collect, to where RegionPredicate is processed.

[gimbling-away]

@rustbot claim

[gimbling-away]

I really apologize for this, I'm mostly busy with life and can't really find the time for working on this, feel free to pick it 💜

[jeremydavis519]

@rustbot claim

[Noratrieb]

PR was closed due to inactivity, feel free to claim it again. @rustbot release-assignment

[LittleFall]

@rustbot claim